The present invention relates to a synchronous detector which is used in a synchronous detection part of a fiber optic gyro, for example, and is particularly suitable for synchronous detection of high frequencies.
A brief description will be given first, with reference to FIG. 1, of the basic construction of an ordinary fiber optic gyro to which the present invention is applicable. Light emitted from a laser or like light source 11 is split by a beam splitter 12 into two beams, which enter, as clockwise and counterclockwise beams 14 and 15, into a loop-like optical transmission line 13 as of a plane-of-polarization retaining optical fiber coil at opposite ends thereof and propagate therethrough in opposite directions. The two 14 and 15 having thus propagated through the optical transmission line 13 and emitted therefrom are coupled again by the beam splitter 12 and interfere with each other. The resulting interference light is converted by an optoelectric transducer 16 into an electric signal corresponding to its intensity, which is cut off in DC and amplified in AC by an AC amplifier 17. An optical phase modulator 18 is provided between the beam splitter 12 and one end of the optical transmission line 13. A bias signal generator 19 applies to the optical phase modulator 18, as a modulation signal, a bias signal V.sub.B by which the phase difference between the two beams 14 and 15 at the time of their interference in the beam splitter 12 alternate between +.pi./2 rad and -.pi./2 rad at intervals of the time .tau. necessary for the light from the light source 11 to propagate through the optical transmission line 13. In synchronization with this phase shift the output of the AC amplifier is synchronously detected (i.e. multiplied by .+-.1) by a synchronous detector 21 for each time .tau..
The phase difference between the clockwise and counterclockwise light beams 14 and 15 when they interfere and the output of the AC amplifier 17 bear such a relationship as indicated by the curve 22 in FIG. 2. When no angular rate is being applied to the optical transmission line 13, the phase difference .phi. between the clockwise and counterclockwise light beams 14 and 15 is caused by their modulation with the optical phase modulator 18 to go positive and negative alternately about the zero phase by the same value at .tau. time intervals as indicated by the curve 23 in FIG. 2. In this instance, the output of the AC amplifier 17 becomes constant as indicated by the line 24 and the output of the synchronous detector 21 is zero. When an angular rate is applied to the optical transmission line 13, a phase difference (i.e. a Sagnac shift) .phi..sub.R develops, owing to the Sagnac effect, between the clockwise and counterclockwise light beams 14 and 15 in accordance with the direction and magnitude of the angular rate being applied. Under the influence of the Sagnac shift .phi..sub.R the phase difference .phi. between the beams 14 and 15 goes positive and negative alternately by the same value about a phase shifted from the zero phase by .phi..sub.R at intervals of the time .tau. as indicated by the curve 25 in FIG. 2. In consequence, the output of the AC amplifier 17 varies like a rectangular wave at intervals of the time .tau. as indicated by the curve 26 in FIG. 2. In this case, the amplitude of the output from the AC amplifier 17 corresponds to the Sagnac shift .phi..sub.R and its phase (in-phase or 180.degree. out-of-phase) relative to the bias signal V.sub.B of the bias signal generator 19 represents the direction of the Sagnac shift. Thus the output waveform 26 of the AC amplifier 17 is detected (multiplied by +1 and -1 alternately every .tau. time) in synchronization with the bias signal V.sub.B and the detected output is provided as a signal representative of the magnitude and direction of the Sagnac shift .phi..sub.R between the clockwise and counterclockwise beams 14 and 15 caused by the angular rate applied to the optical transmission line 13.
The relationship between the input angular rate .omega. and the Sagnac shift .phi..sub.R is expressed by the following equation (1): EQU .phi..sub.R =4.pi.RL.OMEGA./(.lambda.C) (1)
where R is the radius of the optical transmission line (i.e. the optical fiber) 13, L is the length of the optical transmission line 13, .lambda. is the wavelength of the light emitted from the light source 11, C is the velocity of light in a vacuum, and .OMEGA. is the input angular rate.
An angular rate calculator 27 detects the magnitude of the Sagnac shift .phi..sub.R from the output of the synchronous detector 21 and calculates and outputs the input angular rate .OMEGA..
In the conventional fiber optic gyro, an offset contained in the output of the synchronous detector 21 remains intact in the results of measurement of the input angular rate, but since a typical demodulation frequency (determined by 1/(2.tau.)) is relatively high i.e., hundreds of kilohertz to several megahertz, it is difficult in the prior art to design a fiber optic gyro which affords sufficient reduction of the offset of the synchronous detector 21. Ordinary synchronous detectors as well as that for the fiber optic gyro pose the same problem, namely that a relatively large offset develops when the demodulation (i.e. detection) frequency is high.